JP2014099495A - Silicon carbide semiconductor device manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method of a MOS semiconductor device which has a low interface state density at an SiO-SiC interface in a simple and inexpensive method.SOLUTION: In a manufacturing method, a MOS structure is formed by combining extremely simple and highly controllable manufacturing processes such as a process of exposing a surface of a silicon carbide semiconductor to plasma generated inside an ECR sputtering device, a process of depositing an SiOfilm by an ECR sputtering method while maintaining a vacuum state in the same device, a subsequent heat treatment process at approximately 1000°C-1200°C and a process of stacking metal films.

Description

  The present invention relates to a method for manufacturing a silicon carbide (SiC) semiconductor device, and more particularly to a method for manufacturing a semiconductor device having a metal-insulator-semiconductor (MOS) structure having a low interface state density on the surface of a silicon carbide semiconductor layer.

  Silicon carbide has an excellent physical property value of about three times the thermal conductivity and about twice the saturation electron drift velocity as compared with silicon (Si) single crystal widely used as a semiconductor integrated circuit. However, field-effect transistors (MOSFETs) having a MOS structure having characteristics exceeding the material limit of silicon have been researched and developed.

In a MOSFET (SiC-MOSFET) made of silicon carbide, the interface characteristics between the gate insulating film and SiC greatly affect the element characteristics. For example, silicon carbide, like silicon, can form silicon dioxide (SiO ₂ ) by a thermal oxidation method, and the thermal oxidation method is also employed in the manufacturing process of the SiC-MOSFET. However, it is known that many interface states exist at the SiO ₂ -SiC interface only by the thermal oxidation method. In particular, in a MOSFET using SiO ₂ formed by thermal oxidation as a gate insulating film, channel mobility becomes extremely lower than mobility in the bulk due to the interface state close to the conduction band, and on-resistance increases. As a result, there was a problem that the device characteristics deteriorated.

Therefore, in order to reduce the interface state of the SiO ₂ —SiC interface formed by the thermal oxidation method, nitrogen monoxide (NO) or dinitrogen monoxide (NO) is compared with SiC having a (0001) Si surface crystal orientation. N ₂ O) interface (Patent Document 1) or a surface crystal orientation of (000-1) SiC having a C-plane (H ₂ atmosphere) (Patent Document 2) Attempts have been made to reduce the level density.

JP 2005-223003 A JP 2006-196713 A

In the methods described in Patent Documents 1 and 2, carbon atoms are precipitated in the thermal oxide film by high-temperature heat treatment, and interface states are induced, or the reliability of the oxide film is lowered. there were. Furthermore, the thermal oxidation method and the subsequent high-temperature heat treatment require switching of the atmospheric gas and strict control of the temperature profile, and it is necessary to perform heat treatment for a long time, resulting in an increase in manufacturing cost. there were. Furthermore, there has been a problem that the effect of reducing the interface state density improved by the predetermined treatment is deteriorated by the heat treatment in the subsequent manufacturing process. An object of the present invention is to provide a method for manufacturing a semiconductor device having a MOS type structure in which the interface state density at the SiO ₂ —SiC interface is low by solving the above-described problems and using a simple and inexpensive method.

  In order to achieve the above object, a method of manufacturing a silicon carbide semiconductor device according to the present invention includes a plasma generated in an ECR sputtering apparatus on a silicon carbide semiconductor layer surface laminated on a semiconductor substrate surface in an atmosphere containing at least nitrogen gas. Exposing the semiconductor substrate, then depositing a silicon dioxide film on the surface of the silicon carbide semiconductor layer by ECR sputtering while maintaining a vacuum, and heat-treating the semiconductor substrate on which the silicon dioxide film is deposited. And a step of forming a MOS structure electrode by laminating a metal film on the surface of the heat-treated silicon dioxide film.

  According to the manufacturing method of the present invention, a silicon dioxide film can be formed by an ECR sputtering method capable of stacking a dense film in a short time and with good controllability without damaging the plasma processing step and the semiconductor layer continuously performed thereto. By performing the process and the heat treatment process in combination, it becomes possible to easily and inexpensively manufacture a semiconductor device having a MOS structure with excellent characteristics.

  Further, according to the manufacturing method of the present invention, the MOS type structure is formed after the heat treatment at 1000 ° C. to 1200 ° C., so that the characteristics are not deteriorated by the heat treatment performed in the subsequent manufacturing process, and the reliability is high. A semiconductor device can be formed.

It is explanatory drawing of the MOS diode of the 1st Example of this invention. It is explanatory drawing of the manufacturing process of the electrode of the MOS type structure of this invention. It is a graph of the measurement result of the CV characteristic of the MOS diode of this invention. It is a graph which shows the correlation of the interface state density and energy level for investigating the effect of a plasma processing. It is explanatory drawing of the manufacturing process of the vertical MOSFET of 2nd Example of this invention.

The present invention includes a step of exposing a silicon carbide semiconductor surface to plasma generated in an ECR sputtering apparatus, a step of depositing a SiO ₂ film by an ECR sputtering method while maintaining a vacuum state in the same apparatus, and subsequent heating. A MOS type structure can be formed by combining a processing process and a process of laminating metal films, which are very simple and have good controllability. Hereinafter, the manufacturing method of the silicon carbide semiconductor device of this invention is demonstrated in detail.

First, a method for manufacturing a silicon carbide semiconductor device of the present invention will be described in detail by taking a MOS diode as an example. FIG. 1 is a cross-sectional view of a MOS diode, and FIG. 2 is an explanatory view of a manufacturing process of an electrode having a MOS type structure. First, an epitaxial layer 2 made of n-type SiC is formed on an SiC substrate 1 having a surface crystal orientation of (000-1) C-plane 4H—SiC using an epitaxial growth method. The epitaxial layer 2 has a thickness of about 1 to 20 μm and an impurity concentration of about 1 × 10 ¹⁵ to 1 × 10 ¹⁷ cm ⁻³ .

Next, the surface of the epitaxial layer 2 is washed with an organic solvent, hydrofluoric acid or the like to remove impurities and oxide films adhering to the surface, and then the substrate is placed in an ECR sputtering apparatus. Nitrogen (N ₂ ) gas is introduced into the ECR sputtering apparatus, and when the predetermined degree of vacuum is reached, the introduced nitrogen is turned into plasma and the surface of the epitaxial layer 2 is subjected to plasma treatment (S1). This plasma treatment is, for example, a step of exposing the surface of the epitaxial layer 2 for 30 minutes to nitrogen plasma generated under conditions of a nitrogen gas flow rate introduced into the apparatus of 6 sccm, a degree of vacuum of 0.03 Pa, and a microwave power supply voltage of 500 W. . By this plasma treatment, impurities such as oxide and carbon remaining on the surface of the epitaxial layer 2 are removed and cleaned. In this plasma treatment, in addition to nitrogen gas, argon (Ar) or a mixed gas of oxygen (O ₂ ) and nitrogen gas can be used. It is also possible to perform the treatment using different mixed gases a plurality of times.

Next, the SiO ₂ film 3 is deposited on the surface of the epitaxial layer 2 subjected to the plasma treatment without breaking the vacuum degree of the ECR sputtering apparatus (S2). By continuously performing the plasma treatment and the deposition process of the SiO ₂ film 3, it is possible to deposit the SiO ₂ film while maintaining the cleaned surface state. In the SiO ₂ film deposition process, for example, using a silicon target, the argon gas flow rate is 15 sccm, the oxygen gas flow rate is 7 sccm, the degree of vacuum is 0.10 Pa, the microwave power supply voltage is 500 W, the radio frequency (RF) power supply power is 500 W, and the room temperature is 3 When the treatment is performed for 50 minutes, a SiO ₂ film 3 of 50 nm can be deposited.

Next, the SiC substrate is taken out from the ECR sputtering apparatus and heat-treated in a high temperature furnace (S3). This heat treatment is performed at 1200 ° C. for 2 hours in a nitrogen atmosphere. By this heat treatment, chemical bonding is promoted between the deposited SiO ₂ film 3 and SiC on the surface of the epitaxial layer 2, and an SiO ₂ —SiC interface structure having a low interface state density can be formed. Here, since the heat treatment is performed in a nitrogen atmosphere, unlike the general thermal oxidation method, defects caused by carbon (C) escaping from the SiC surface by oxidation or carbon diffuses into the SiO ₂ film. As a result, the SiO ₂ film is not deteriorated and the reliability is not lowered. Note that the atmospheric gas can be changed to argon (Ar) or a mixed gas of hydrogen (H ₂ ) gas and nitrogen gas. Further, the temperature of the heat treatment must be set to a temperature of at least 1000 ° C. or higher for improving the MOS interface state, and set to a temperature not exceeding the melting point of silicon dioxide of 1400 ° C. In particular, as described above, heat treatment at 1200 ° C. has a large effect of improving characteristics.

Next, in order to form an anode electrode of a diode on the SiO ₂ film 3, an aluminum film is deposited. As a result, a metal film is formed on the epitaxial layer 2 via the SiO ₂ film 3 to complete the MOS structure (S4). Finally, in order to form the cathode electrode 5 of the diode on the back surface of the SiC substrate 1, an aluminum film is deposited and a MOS diode is completed.

FIG. 3 is a graph showing the measurement results of the CV characteristics of the MOS diode formed by the above manufacturing method. The measurement was performed by applying a voltage of −10 to +10 V to the anode electrode at a measurement frequency of 100 kHz. For comparison, a SiO ₂ film was deposited after plasma treatment and measured without heat treatment (no heat treatment), and a SiO ₂ film was deposited without plasma treatment and was measured by heat treatment ( (Plasma treatment not performed) is also displayed.

As shown in FIG. 3, it can be seen that without the heat treatment, the CV characteristic does not increase to the oxide film capacity (C _OX ) even when entering the accumulation region. On the other hand, it can be seen that the oxide film capacitance is observed and the state of the MOS interface is improved without the plasma treatment in which only the heat treatment is performed, compared with the case without the heat treatment. In comparison with this, it can be seen that the capacitance change of the CV characteristic becomes steep in the present invention, and the characteristic improvement is further advanced.

Next, in order to confirm the effect of the presence or absence of plasma treatment, the interface state density (Dit) was measured by the High-Low C-V method. FIG. 4 is a graph showing the correlation between the interface state density and the energy level. The interface state density was estimated from the capacitance difference between the high-frequency CV characteristic (measurement frequency 100 kHz) and the quasi-qualitative CV characteristic (step voltage 50 mV, delay time 1 sec). As shown in FIG. 4, it can be seen that the plasma state of the present invention is lower in the interface state density and the MOS interface state is improved than the case where the plasma treatment is not performed. In particular, when comparing the interface state density in the vicinity of the energy level (Eit = 0.2 eV) in the vicinity of the conduction band affecting the carrier mobility, it is 5.2 × 10 ¹² cm ⁻² · eV in the case of no plasma treatment. ^In contrast, when the plasma treatment was performed, it was 6.2 × 10 ¹¹ cm ⁻² · eV ⁻¹ , and it was confirmed that the interface state density was reduced by about one digit.

  In this example, 4H-SiC having a (000-1) C plane crystal orientation on the surface of the SiC substrate was used, but (0001) Si plane 4H-SiC, or 6H-SiC having a different polytype, Similar effects can be obtained with 3C-SiC.

Next, a vertical MOSFET manufacturing method will be described as a second embodiment. First, an epitaxial layer 2 made of n-type SiC is formed on an SiC substrate 1 having a surface crystal orientation of (000-1) C-plane 4H—SiC using an epitaxial growth method. The epitaxial layer 2 has a thickness of about 1 to 20 μm and an impurity concentration of about 1 × 10 ¹⁵ to 1 × 10 ¹⁷ cm ⁻³ .

  Next, the surface of the epitaxial layer 2 is washed with an organic solvent, hydrofluoric acid, etc., and after removing impurities and oxide films adhering to the surface, a photoresist is used as an implantation mask to form a base region. Aluminum (Al) is ion-implanted on the surface to form a base region 6 made of a p-type diffusion region. Further, in order to form a source region and a drain region in the base region 6, another photoresist is used as an implantation mask, phosphorus (P) is ion-implanted into the surface of the epitaxial layer 2 in which the base region 6 is formed, and n A source region 7 made of a mold diffusion region is formed. The implanted impurity ions are activated by heat treatment at 1650 ° C. for 1 minute, for example (FIG. 5a).

Next, as in the first embodiment, the surface of the epitaxial layer 2 is washed with an organic solvent, hydrofluoric acid or the like to remove impurities and oxide films adhering to the surface, and then placed in an ECR sputtering apparatus. Nitrogen (N ₂ ) gas is introduced into the ECR sputtering apparatus, and when the predetermined degree of vacuum is reached, the introduced nitrogen is turned into plasma, and the surface of the epitaxial layer 2 is plasma treated. This plasma treatment is performed, for example, by exposing the surface for 30 minutes to nitrogen plasma generated under the conditions of a flow rate of nitrogen gas introduced into the apparatus of 6 sccm, a degree of vacuum of 0.03 Pa, and a microwave power supply voltage of 500 W. By this plasma treatment, impurities such as oxide and carbon remaining on the surface are removed and cleaned. In this plasma treatment, in addition to nitrogen gas, argon (Ar) or a mixed gas of oxygen (O ₂ ) and nitrogen gas can be used. It is also possible to perform the treatment using different mixed gases a plurality of times.

Next, the SiO ₂ film 3 is deposited on the surfaces of the epitaxial layer 2, the base region 6 and the source region 7 that have been subjected to the plasma treatment without continuously breaking the degree of vacuum of the ECR sputtering apparatus. By continuously performing the plasma treatment and the deposition process of the SiO ₂ film 3, it is possible to deposit the SiO ₂ film while maintaining the cleaned surface state. In the SiO ₂ film deposition process, for example, using a silicon target, the argon gas flow rate is 15 sccm, the oxygen gas flow rate is 7 sccm, the degree of vacuum is 0.10 Pa, the microwave power supply voltage is 500 W, the radio frequency (RF) power supply power is 500 W, and the room temperature is When the treatment is performed for 3 minutes, a 50 nm SiO ₂ film 3 can be deposited. This SiO ₂ film 3 becomes a gate insulating film. Next, the SiC substrate is taken out from the ECR sputtering apparatus and heat-treated in a high temperature furnace. This heat treatment is performed in a nitrogen atmosphere at 1200 ° C. for 2 hours to form a SiO ₂ —SiC bond (FIG. 5b).

A polycrystalline silicon (polysilicon) film is formed on the SiO ₂ film 3 by chemical vapor deposition (CVD), and patterned into a predetermined shape, thereby forming a gate electrode 8. Thereafter, the gate insulating film 9 is left, the SiO ₂ film 3 on the source region 6 is removed, the surface of the source region 7 is exposed, and a source electrode 10 made of nickel (Ni) is formed on the exposed surface of the source region 7. . Furthermore, in order to form a drain electrode on the back surface of the SiC substrate 1, a drain electrode 11 made of nickel is formed. In order to form an ohmic junction between the source electrode 7 and the drain electrode 11, a heat treatment is performed at 950 ° C. for 2 minutes in a nitrogen atmosphere to complete a vertical MOSFET (FIG. 5c). This heat treatment condition is that the heating temperature is lower than the heat treatment condition when the SiO ₂ -SiC MOS structure is formed after the SiO ₂ film 3 is deposited and the heating time is shorter, so that the interface of the previously formed MOS structure The state does not deteriorate.

  Also in this embodiment, since the MOS structure with a low interface state density is formed as in the first embodiment described above, the formed vertical MOSFET has increased channel movement, reduced on-resistance, and switching operation. The power conversion efficiency (power loss) in can be improved.

1: SiC substrate, 2: epitaxial layer, 3: SiO ₂ film, 4: anode electrode, 5: cathode electrode, 6: base region, 7: source region, 8: gate electrode, 9: gate insulating film, 10: source Electrode, 11: drain electrode

Claims (1)

Exposing the silicon carbide semiconductor layer surface laminated on the semiconductor substrate surface to plasma generated in an ECR sputtering apparatus in an atmosphere containing at least nitrogen gas;
Then, a step of depositing a silicon dioxide film on the surface of the silicon carbide semiconductor layer by ECR sputtering while maintaining a vacuum state;
Heat-treating the semiconductor substrate having a silicon dioxide film deposited on the surface;
And a step of laminating a metal film on the surface of the heat-treated silicon dioxide film to form an electrode having a MOS structure.

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